(1) Field of the Invention
This invention relates to an optical material, and specifically to an optical material formed of a copolymer containing 4-isopropenylbiphenyl and a particular monofunctional aromatic monomer as essential components and having a high refractive index.
(2) Description of the Related Art
Inorganic glass is now used widely as an optical material for lenses, prisms and the like. Since the inorganic glass employed as an optical material has a specific gravity as high as 2.4-6.2, the optical material has a heavy weight, thereby encountering a problem upon incorporation into optical systems such as office automation equipment for which size and weight reduction is essential. With a view toward solving the problem, a great deal of work has been carried out for the development of optical materials having a lower specific gravity compared to inorganic glass in general and formed of a polymer or copolymer. Some optical materials of the above-mentioned sort have already found actual utility, for example, as spectacle lenses.
A variety of characteristics are required for such optical materials. Among these, high refractive index, low specific gravity, colorlessness and excellent transparency can be mentioned fundamentally as important characteristics. When an optical material to be used as a spectacle lens has a high refractive index and a low specific gravity, the peripheral thickness of the lens can be reduced substantially, so that the lens can be provided as a thin lens as a whole. A weight reduction is therefore feasible from this point too. Such an optical material is thus extremely preferred.
However, in case of obtaining a lens from an optical material having a low specific gravity and a low refractive index, it is necessary to reduce the radius of the lens in order to provide a necessary refracting power. As a result, both the thickness and the volume of the lens are increased. Although the weight reduction was sought for, this objective cannot be achieved fully as a consequence. When an optical material is itself colored or has inferior transparency, use of a lens or the like made of such optical material in an optical system tends to result in a problem such that the transmission spectrum is distorted or the transmittance of light of a desired wavelength is lowered. A limitation is therefore imposed on its application field. Such an optical lens is hence not desired.
As optical materials having a low specific gravity, polymethyl methacrylate (specific gravity, d: 1.19, refractive index n.sub.d : 1.49), polydiethylene glycol bisallylcarbonate (d: 1.32, n.sub.d : 1.50), polycarbonate (d: 1.20, n.sub.d : 1.58), polystyrene (d: 1.06, n.sub.d : 1.59) are known widely these days. None of these optical materials however have any sufficiently high refractive index.
On the other hand, a variety of proposals has been made on optical materials having a high refractive index. For example, Japanese Patent Publication No. 14449/1983 discloses dimethacrylate or diacrylate copolymers in which each nucleus halogen-substituted aromatic ring is coupled to a methacryloyloxy or acryloyloxy group through an alkylene glycol group. Further, Japanese Patent Laid-Open No. 51706/1985 discloses polymers of a urethanated (meth)acrylic monomer formed by reacting a bromine-substituted aromatic monomer having one or more hydroxyl groups with a polyfunctional isocyanate.
In order to obtain an optical material having sufficiently high refractive index in accordance with these techniques, it is however indispensable to increase the proportion of the halogen atoms contained. This has lead to a problem that the specific gravity of the optical material becomes high. For example, optical materials of this sort whose refractive indexes n.sub.d are as high as at least 1.60 have a specific gravity d as high as 1.4-2.2 without exception.
Some proposals have also been made regarding optical materials having a high refractive index and a low specific gravity. None of these proposals are however fully satisfactory. Namely, they are accompanied by one or more problems such that they are colored or they are susceptible to coloration or transparency reduction due to their low chemical stability and their production is not easy due to poor solubility of raw materials. There is another problem that the wavelength dependency (dispersion characteristics) of their refractive indexes is great and a limitation is hence imposed on their application as optical materials.